1. Field of the Invention
The present general inventive concept relates to a light scanning unit which scans light emitted by a light source to a surface to be emitted and an image forming apparatus employing the same, and more particularly, to a light scanning unit which narrows light amount ratio differences which occur due to polarization properties of incident light and transmittance differences depending on a deflection angle of a plastic lens, and an image forming apparatus employing the same.
2. Description of the Related Art
Generally, a light scanning unit is employed in an image forming apparatus such as a laser printer, a digital printer and a facsimile, and a bar code reader. The light scanning unit forms a latent image on a photosensitive body by a main scanning of a beam deflector and a sub-scanning by rotation of the photosensitive body.
FIG. 1 illustrates an optical arrangement of a conventional light scanning unit which is disclosed in U.S. Pat. No. 7,068,406B2 issued on Jun. 27, 2006.
As illustrated in FIG. 1, the conventional light scanning unit includes a laser diode 1 which emits light beam, a polygon mirror 5 which deflects incident light to scan the light beam emitted by the laser diode 1 to a photosensitive body 8, and f-θ lenses 6 and 7 which correct errors included in the light beam deflected by the polygon mirror 5. A collimating lens 2, a stop 3, and a cylindrical lens 4 are provided in a light path between the laser diode 1 and the polygon mirror 5. The collimating lens 2 focuses light emitted by the laser diode 1 and the cylindrical lens 4 shapes light.
The light scanning unit employs the laser diode 1 which emits light beam having a short wavelength, approximately 500 nm or less, to reduce a spot size of the scanned light beam. The light scanning unit uses at least one f-θ lens which includes plastic, thereby lowering production costs thereof.
In this case, the amount of light may vary due to light absorption differences within the plastic lens. That is, the transmittance varies due to different light beam passing distances according to a deflection angle from the light axis of the plastic lens, thereby causing difference of light amount ratio in a main scanning direction. Then, the latent image which is formed on the photosensitive body 9 becomes ununiform.
To solve such a problem, the thickness of the conventional f-θ lens including plastic is set to satisfy Formula 1 or Formula 2 below.L1−L2<10 mm  [Formula 1]
L1 and L2 respectively represent a maximum value and a minimum value of a total light beam passing distance depending on the deflection angle from the light axis of at least one plastic lens.L1−L2<3·log(0.93)/S S=log(1−3.55×108/λ4)  [Formula 2]
Here, λ is a wavelength of the light beam emitted by the laser diode 1.
The light scanning unit includes the plastic lens whose L1 and L2 are set to satisfy the Formula 1 or 2 regardless of a rotating direction of the laser diode 1.
The laser diode 1 has such a characteristic that it emits predetermined polarized light, which is changed depending on the arrangement direction. The transmittance difference of the f-θ lens including plastic is approximately 10% in the main scanning direction according to the polarization properties of the light emitted by the laser diode 1.
If the polarization properties of the laser diode 1 are not considered, errors are generated on a large scale and the degree of freedom in an optical design is distorted due to the transmittance differences of the plastic lens.